Replace the Out_of_node_handles exception with Out_of_metadata.
Clients need to know when the server is out of internal resources,
but not why.
Cleanup and sort the errors at file_system_session.h.
Remove 'Size_limit_reached exception' from File_system, which was
internal to ram_fs.
Issue #1751Fixes#1909
This patch changes the top-level directory layout as a preparatory
step for improving the tools for managing 3rd-party source codes.
The rationale is described in the issue referenced below.
Issue #1082
